This invention relates to an absolute position detecting device which is adapted to detect an absolute position of an endless rotating device such as an endless conveying device in an automatic tool exchanging device.
An automatic tool exchanging device of the type wherein a plurality of tools are rotated by an endless conveyance means such as an endless chain (herein referred to as "a chain") is well known in the art. In such automatic tool exchanging devices, the absolute position of the endless conveyance means, with respect to its starting point of revolution (or point of origin, or initial point), is determined by a controller that counts the number of revolutions and detects the absolute position within one revolution of a driving servo motor, so that the position of the next tool to be used can be indexed. In this manner, out of the plurality of tools, a targeted tool can be selected.
When the device is stopped because of a power supply failure, the absolute position detecting device is activated by an auxiliary power source, namely, a battery, to detect the position data of the device. The position data thus detected is then read by the controller when the power supply is restored. Thus, even if the device is stopped by a power supply failure, its remaining operation can be carried out successively when the power supply is restored.
FIG. 1 shows one example of a conventional absolute position detecting device as just described. In FIG. 1, reference numeral 1 designates a number-of-revolutions sensor which is coupled to a servo motor; reference numeral 2 designates a number-of-revolutions counter for counting the number-of-revolutions of the motor from the output signal of the sensor 1; and reference number 3 designates an absolute-position-within-one-revolution sensor. The output signals of the counter 2 and the absolute sensor 3 are applied to an absolute data register 4 whose output is then applied through a modem 5 to the above-described controller.
In the conventional absolute position detecting device thus described, the number of revolutions counted can accumulate to a point such that the number-of-revolutions counter 3 reaches its maximum count. This is a problem. In the case of a so-called "short-cut position indexing operation", a similar problem can arise. That is, when the device is used for a long period of time, the count value may be greatly increased in one of the directions of circulation, resulting in an overflow in the number-of-revolutions counter 3. It is, therefore, impossible to obtain the absolute position data.